In recent years, nonaqueous electrolyte secondary batteries having a high energy density and excellent in cycle characteristics have drawn attention as an electric power source for mobile phones, portable appliances such as notebook computers, and electric vehicles. Among such nonaqueous electrolyte secondary battery, presently most widely commercialized ones are compact and consumer use type batteries having 2 Ah mainly mobile phones.
At present, various types of positive active materials for nonaqueous electrolyte secondary batteries are made available. Most commonly known ones are lithium-containing transition metal oxides having basic configuration of lithium cobalt oxide (LiCoO2) and lithium nickel oxide (LiNiO2) having an operation voltage of around 4 V, lithium manganese oxide (LiMn2O4) having spinel structure, and the like. Among these, lithium cobalt oxide has been widely employed as a positive active material since it is excellent in the charge-discharge characteristics and energy density in small capacity lithium secondary batteries with a battery capacity to 2 Ah.
However, in consideration of size enlargement to middle to large scale of batteries from now on, particularly development of batteries' applications for industrial uses, which are expected to become high demand, safety of batteries is very important issue. Accordingly, the configurations of presently available compact batteries cannot satisfy the required safety. One of reasons for that is that the thermal stability of the positive active materials of them is low.
Therefore, recently, lithium iron phosphate having an olivine structure with high thermal stability has drawn attention as a positive active material. Since this lithium iron phosphate has a covalent bond of phosphorus and oxygen, it does not release oxygen even at a high temperature. Accordingly, use of lithium iron phosphate as a positive active material can remarkably improve the safety of batteries.
However, it is known that since lithium iron phosphate has a low electron conductivity, a battery using it as an active material is inferior in the high rate discharge characteristic.
Further, as described in Japanese Patent Application Laid-Open (JP-A) No. 2001-250555 (Patent Document 1), which is a Japanese patent document, hydrated ferrous salt particles, which are a raw material, are very hard due to crystal growth and difficult to be processed such as pulverization and it is possible to obtain only coarse particles with an average particle diameter of several to several tens μm. The average particle diameter of lithium iron phosphate (LiFePO4) obtained by firing a mixture of such a coarse hydrated ferrous salt and fine lithium phosphate becomes several μm or higher.
As described, in addition to the coarse size of the hydrated ferrous salt particles, since reactivity of lithium iron phosphate in chemical reaction is low, lithium iron phosphate with an uneven composition in particle level tends to be synthesized easily. Therefore, a nonaqueous electrolyte secondary battery using the lithium iron phosphate as a positive active material tends to have a low capacity.
Patent Documents 2 to 10 disclose means for forming carbon coat on the surface of lithium iron phosphate particles in order to improve the above-mentioned defective point that the lithium iron phosphate has low electron conductivity.
JP-A No. 2001-015111 (Patent Document 2), which is a Japanese patent document, discloses a technique of obtaining lithium iron phosphate (LiFePO4) coated with a carbonaceous supported material. As the coating technique is disclosed that a mixture of vivianite (Fe3(PO4)2.8H2O), lithium orthophosphate, and a polypropylene powder is finely pulverized by a zirconia ball mill and then heated at 350° C. to 700° C.
In this document, that carbon is supported by heterogeneous reaction of carbon monoxide is also disclosed. Further, this document discloses examples such as polyvinyl alcohols, phenol condensation products, furfuryl alcohol-derived polymers as a precursor material for a carbonaceous material.
JP-A No. 2002-117833 (Patent Document 3), which is a Japanese patent document, discloses a method of obtaining LiFePO4/carbon composite. As a method, the document discloses that a mixture obtained by further adding an acetylene black powder to a mixture of Li3PO4 and Fe3(PO4)2.8H2O is milled by a planetary ball mill and fired at 600° C.
JP-A No. 2003-034534 (Patent Document 4), which is a Japanese patent document, discloses a method for producing a carbon-containing lithium iron oxide for a positive active material for lithium secondary batteries by compounding particles of lithium iron oxide (LiFePO4) with olivine structure with carbonaceous fine particles. As the production method, the document discloses that a mixture obtained by mixing a lithium compound, an iron compound, a phosphorus-containing ammonium salt, and carbonaceous fine particles is fired at a temperature of 600° C. to 750° C.
JP-A No. 2003-292308 (Patent Document 5), which is a Japanese patent document, discloses a method for producing lithium iron phosphorus composite oxide carbon obtained by coating LiFePO4 particle surface with a conductive carbon material. As the production method, the following technique is disclosed. A mixture is prepared by mixing ferrous phosphate hydrate (Fe3(PO4)2.8H2O), lithium phosphate (Li3PO4), and the conductive carbon material. This mixture is pulverized in dry manner to obtain a reaction precursor with a specific volume of 1.5 ml/g or less. This reaction precursor is fired to coat particle surface of LiFePO4 with the conductive carbon material and then the resulting product is pulverized.
JP-A No. 2004-186075 (Patent Document 6), which is a Japanese patent document, discloses a technique of coating the surface of lithium iron oxide, which is a positive active material for nonaqueous electrolyte secondary batteries with carbon fibers.
Japanese Patent Application National Publication No. 2004-509058 (Patent Document 7), which is a Japanese patent document, discloses the following two methods as a method for coating LiFePO4 with carbon. One is a method of heating a mixture prepared by mixing LiFePO4 with acetic acid cellulose as a carbon precursor in the presence of argon ambient current. The other is a method of heating a raw material of LiFePO4 together with polyvinyl alcohol as a carbon source. Further, this document exemplifies a polymer of furfuryl alcohol as an organic substance for conductive carbon source.
Japanese Patent Application National Publication No. 2004-509447 (Patent Document 8), which is a Japanese patent document, discloses the following two methods as a method for coating LiFePO4 with carbon. One is a method of heating a mixture prepared by mixing LiFePO4 with acetic acid cellulose as a carbon precursor in the presence of argon atmospheric air flow. The other is a method of heating a raw material of LiFePO4 in the presence of ethylene oxide-containing polyethylene-block-poly(ethylene glycol) type carbon additive while a CO/CO2 mixed gas is circulated. Further, this document exemplifies a polymer of furfuryl alcohol as an organic substance for a conductive carbon source.
US Patent Application Laid-Open No. 2004/0157126 (Patent Document 9) discloses a synthesis method of a positive active material by coating cores of LiFe1-xMxPO4 (M is selected from the group consisting of Mn, Co, Ti, and Ni and 0≦x≦1) with carbon by thermal decomposition of a hydrocarbon-containing gas mixture as a carbon source.
JP-A No. 2003-292309 (Patent Document 10), which is a Japanese patent document, discloses a production method of LiFePO4 coated with a carbonaceous material. As the production method, the document discloses that a mixture of a raw material of LiFePO4 and polyethylene glycol with an average molecular weight of 1900 to 2100 is fired in nitrogen atmosphere. Further, this document exemplifies furfuryl alcohol resin and polyvinyl alcohol as a carbonaceous material precursor, which is an organic compound to be carbonized by firing.
Patent Document 1: JP-A No. 2001-250555
Patent Document 2: JP-A No. 2001-015111
Patent Document 3: JP-A No. 2002-117833
Patent Document 4: JP-A No. 2003-034534
Patent Document 5: JP-A No. 2003-292308
Patent Document 6: JP-A No. 2004-186075
Patent Document 7: Japanese Patent Application National Publication No. 2004-509058
Patent Document 8: Japanese Patent Application National Publication No. 2004-509447
Patent Document 9: US Patent Application Laid-Open No. 2004/0157126
Patent Document 10: JP-A No. 2003-292309